You can also use it the other way. You can use it in a passive ambient way and just stick it into the environment, but they suggest otherwise to get the most accurate determination correlating with the actual subject.
I'm using the manufacturer's information here. This is not data that we've developed. Scientists are by nature skeptics. We like to have our own data. We test these things because we don't believe anybody until we check it for ourselves.
This manufacturer says two inches. I realize that the original data that came from NHTSA back in the late 1980s suggested six inches. That is a different device entirely. It maybe just illustrates for this committee that these devices are very different from one another and it depends on the manufacturer and which device we're testing as to how we draw our conclusions.